I don't get why skin mods conflict though. Others have plenty of mods & run local servers with add-on maps perfectly fine.

I honestly have no idea how it works, but it's something that can conflict. When playing in Windows 7 I could host easily with all my mods running, but in Win 8 I have to disable everything so people can join. Even if your server isn't enforcing consistency, player mods that conflict with server mods can give you trouble. If you can host without your mods, you might just have to do that.